A concern for future diesel fuels is the ability to use low value, high emissions materials currently produced in refineries in higher quality diesel fuels without extensive and expensive reprocessing. These materials typically have high density, may have high end boiling and T95 points, (the temperature at which most all the material has boiled off, leaving only 5% remaining in the distillation pot) high aromatic and polyaromatic contents and high sulfur contents. These factors have been shown to have a detrimental effect on emissions. For example, see the Coordinating Research Council (CRC) study on heavy duty diesels in the United States reported in SAE papers 932735, 950250 and 950251, and the European Programme on Emissions, Fuels and Engine Technologies (EPEFE) study on light and heavy duty diesels reported in SAE papers 961069, 961074 and 961075.
Particularly, increases in aromatic content of fuels have been cited as having a negative impact on emissions, see ASTM D 975-98b. As a result, the California Air Resources Board (CARB) mandated a maximum aromatics content for commercial diesel fuels of 10 volume % (9.5 wt %), see SAE Paper 930728. However, CARB permits some high aromatic and polyaromatic diesel 930728. However, CARB permits some high aromatic and polyaromatic diesel fuels to be produced and sold if it can be established that the higher aromatic and polyaromatic diesel fuel has combustion emissions properties at least equivalent to those of a standard 10 vol. % max aromatic fuel. See Subsection (g) of Section 2282, Title B, California Code of Regulations
In contrast, emissions measurements on Fischer-Tropsch diesel fuels, which have virtually nil sulfur, aromatic and polyaromatic contents demonstrate favorable emissions characteristics. A report by the Southwest Research Institute (SwRI) entitled “The Standing of Fischer-Tropsch Diesel in an Assay of Fuel Performance and Emissions” by Jimell Erwin and Thomas W. Ryan, III, NREL (National Renewable Energy Laboratory) Subcontract YZ-2-113215, October 1993, details the advantage of Fischer-Tropsch fuels for lowering emissions when used neat, that is, use of pure Fischer-Tropsch diesel fuels.
Thus, there remains a need to develop a superior economic fuel blend useful as a diesel fuel while lowering emissions after combustion. In particular, emissions of solid particulate matter (PM) and nitrogen oxides (NOx) are especially important due to current and proposed environmental regulation. In this regard, the ability to incorporate cracked stocks in diesel fuels while maintaining emissions standards will provide a distinct economic advantage.
By virtue of the present invention, Fischer-Tropsch diesel fuels are blended with lower grade cracked stocks to produce a composition useful as a diesel fuel which satisfies current diesel emissions standards. Further, the blend of the present invention can incorporate higher concentrations of both polyaromatics and aromatics while maintaining or exceeding emissions specifications after combustion in a diesel engine.
The citations of the several SAE papers referenced herein are:    P. J. Zemroch, P. Schimmering, G. Sado, C. T. Gray and Hans-Martin Burghardt, “European Programme on Emissions, Fuels and Engine Technologies-Statistical Design and Analysis Techniques”, SAE paper 961069.    M. Signer, P. Heinze, R. Mercogliano and J. J. Stein, “European Programme on Emissions, Fuels and Engine Technologies-Heavy Duty Diesel Study”, SAE paper 961074.    D. J. Rickeard, R. Bonetto and M. Signer, “European Programme on Emissions, Fuels and Engine Technologies-Comparison of Light and Heavy Duty Diesels”, SAE paper 961075.    K. B. Spreen, T. L. Ullman and R. L. Mason, “Effects of Cetane Number, Aromatics and Oxygenates on Emissions from a 1994 Heavy-Duty Diesel Engine with Exhaust Catalyst”, SAE paper 950250.    K. B. Spreen, T. L. Ullman and R. L. Mason, “Effects of Cetane Number on Emissions from a Prototype 1998 Heavy Duty Diesel Engine”, SAE paper 950251.    Thomas Ryan III and Jimell Erwin, “Diesel Fuel Composition Effect on Ignition and Emissions”, SAE paper 932735.    M. Hublin, P. G. Gadd, D. E. Hall, K. P. Schindler, “European Programme on Emissions, Fuels and Engine Technologies-Light Duty Diesel Study”, SAE paper 961073.    Manuch Nikanjam, “Development of the First CARB Certified California Alternative Diesel Fuel”, SAE paper 930728.